1. Field of the Invention
The present invention relates to a method for fabricating a memory device, particularly to a method for fabricating a phase change memory device, which has a smaller contact area between the heater and the phase change material.
2. Description of the Prior Art
The phase change memory device is a non-volatile random access memory. The phase change memory device contains phase change materials which will switch between a crystalline state and a non-crystalline state by applying appropriate current. Different states of the phase change material, such as a crystalline state, a semi-crystalline state or a non-crystalline state, imply different electric resistances. Normally, the non-crystalline state has a higher electric resistance than the crystalline state. Therefore, data is accessible by measuring the electric resistances.
In order to vary the state of a phase change material, a heater is used to heat the phase change material. In a conventional phase change memory device, a larger junction exists between the heater and the phase change material to achieve better electric conduction. However, phase change needs higher power in a phase change material having a larger junction. Besides, the phase change material is likely to have voids during repeated phase changes, which would degrade the reliability. In another conventional phase change memory device, the phase change material is filled into a gradually-shrinking recess so as to reduce the junction between the heater and the phase change material. As the recess has a smaller bottom, the recess is likely to be incompletely filled and thus have voids, which may degrade the reliability or even directly damage the memory cell.
In a conventional method for fabricating a phase change memory device, a larger through-hole is formed in the mask beforehand. Next, an appropriate material is deposited inside the through-hole. During the deposition process, the through-hole is gradually closed to form a void thereinside. Next, the deposited material is etched to open the through-hole and define a smaller through-hole. Finally, the smaller through-hole is used to define a smaller heater. However, the abovementioned semiconductor process is undeveloped and hard to control the size of the void. Thus, the size of the heater is also hard to control. Therefore, the variance among the larger through-holes, the voids and the smaller through-holes must be stringently controlled lest the memory cells have too great a difference therebetween.
Accordingly, the industry is eager to develop a technology to stably fabricate a phase change memory device having higher reliability and smaller contact area between the heater and the phase change material.